1. Field of the Invention
This invention relates to an apparatus for the application of flux. More particularly, it relates to an apparatus for foam flux application, which enables the application of flux on printed boards to be effected by foaming the flux and bringing the printed boards into contact with the foam flux thereby fracturing the foam flux and allowing the scattered flux to land on and coat the printed boards.
2. Description of the Prior Art
In recent years, the process of soldering the printed boards is tending increasingly toward automation owing to the devotion of industrial efforts to operational rationalization. The automatic soldering of printed boards is generally accomplished by causing printed boards to be set fast at the fixed positions on a conveying means, moving the conveying means carrying the printed boards thereon through the interior of the soldering system, performing on the printed boards in motion the sequential steps of flux application, preheating, soldering, cutting lead terminals of part, and cleaning and optionally the further steps of flux application, preheating, finish soldering, and washing. The printed boards on which the soldering operation has been completed are removed from the conveying means. In the meantime, new printed boards are set fast at the fixed positions on the conveying means.
In the automatic soldering system of the kind described above, the application of flux can be carried out by immersing the printed boards brought in as carried on the conveying means into a flux tank. In accordance with this method, if the printed boards are immersed in the tank as conveyed in the horizontal direction, the flux liquid with which the printed boards come into contact exerts fluid resistance upon the printed boards in motion. If the speed of this conveyance is high, there is the possibility that the various electronic components disposed on the printed boards will be separated from the printed boards by this fluid resistance. The immersion, therefore, must be effected by advancing the printed boards on the conveying means downwardly into the flux tank, necessitating use of a complicate conveying mechanism. Further, because of the interfacial tension to be exerted on the faces of contact, uniform application of flux on all the printed boards is attained only with difficulty. There is another hard problem, i.e. the difficulty to be encountered in keeping the level of the flux liquid constant because the level inevitably falls with the use of the flux liquid.
For the purpose of overcoming the drawbacks suffered as described above by the prior art, apparatuses for foam flux application constructed as disclosed in Japanese Patent Laid-Open SHO No. 60(1985)-121,065, for example, have been developed and found actual utility as means of flux application. One foam flux application apparatus comprises, as illustrated in FIG. 6, a flux tank 101, a partition 105 possessing a top portion 103 rising above the fixed level 102 of flux liquid held inside the flux tank 101, containing near the bottom portion thereof a perforated portion 104, and serving to divide the interior of the flux tank 101 into an inner tank portion 106 and an outer tank portion 107, and one or more porous tubes 108 disposed inside the inner tank portion 106 and adapted to communicate with a compressed air feed mechanism (not shown). When compressed air is fed from the compressed air feed mechanism into the porous tube 108, it is blown out of the numerous pores in the porous tube 108 to foam a flux liquid 109. The foamed flux liquid 109 rises inside the partition 105 and forms a foam layer 110 on the empty space (the inner tank portion 106) enclosed with the partition 105. The foam flux liquid 109 which has overflowed the top of the partition 105 flows down the lateral surface of the partition 105 and falls into the outer side (outer tank portion 107) of the partition 105. In the flux tank 101, the inside and the outside of the partition 105 communicate with each other via the perforation 104. When the supply of flux liquid in the inner tank portion 106 falls short, the flux liquid inside the outer tank portion 107 flows through the perforation 104 into the inner tank portion 106 to replenish. In this apparatus, the application of flux on a printed board is effected by moving the printed board in a horizontal direction while allowing the printed board to come into contact with the upper surface of the foam layer 110 formed as described above thereby fracturing the portion of the foam flux with which the printed board has come into contact and allowing the scattered flux to land on and wet the printed board.
When the application of flux is carried out by the use of the conventional foam flux application apparatus, however, the upper side of the foam flux layer does not form a uniform flat surface because it is undulated or compelled to form depressions shaped like craters locally. Thus, there is the possibility that part of the printed board will escape coming into contact with the upper surface of the foam flux layer. At times, the flux, on being foamed, gives rise to giant bubbles. There is the possibility that when the printed board comes into contact with such a giant bubble and fractures this giant bubble, the fine droplets of flux scattered from the fractured giant bubble will fail to land on part of the printed board. In any event, uniform application of the flux on the printed board has been impeded by these phenomena. In the printed boards of the recent development are included those of the type having chips and other similar parts mounted on the surfaces intended for soldering. When the printed boards of this type are to be soldered, the adverse phenomena are posed as serious problems. Since the application of flux is intended to remove a thin film of copper oxide formed on the surface of copper foil of the printed board thereby ensuring intimate adhesion of solider to the copper foil during the course of soldering operation, the failure to effect uniform application of flux to the printed board results in a defection of the soldering operation and an impairement of the economic value of the product to be obtained at all.
A study continued to determine the cause has led to a conclusion that the defective foaming of flux is mainly ascribable the porous tube to be used in the apparatus. The porous tube used in the conventional foam flux apparatus has been made of biscuit or sintered stainless steel. In the case of a porous tube of biscuit, substantially no control is made of pore distribution or pore diameter distribution during the course of the production of the tube. In the case of a tube of sintered metal, the porosity thereof is low. Since this tube is produced by first making a sintered plate and then molding the sintered plate in the shape of a tube, the pore diameters first fixed in the sintered plate are decreased on the inner side of the tube and increased on the outer side of the tube and part of the pores are closed, making the control of pore distribution and pore diameter distribution difficult. As the result, the porous tube fails to generate uniform fine bubbles of flux and entails the problems mentioned above.
Further the porous tube of biscuit suffers from very weak strength, insufficient durability, and heavy dispersion of quality. In the case of the porous tube of sintered metal, since it is kept immersed in the flux, it has the possibility of being corroded by a chemical reaction when the flux is deteriorated to the extent of generating an acid or an alkali. Further, since it is obtained by rolling a sintered plate in the shape of a tube, it must be disposed so that the welded portion will fall on the underside.
Further, the defective foaming of the flux is also ascribable partly to the fact that the flux converted into a sludge by aging enters the pores in the porous tube and solidifies therein during the stop of the operation of the apparatus to alter the pore diameters.
The present invention, therefore, aims to solve the various problems mentioned above.
An object of this invention is to provide an improved apparatus for the application of flux.
Another object of this invention is to provide for an automatic soldering equipment an apparatus for foam flux application capable of uniformly applying flux on a printed board.
Yet another object of this invention is to provide an apparatus for the application of flux which has little possibility of yielding defective products during the course of soldering of printed boards.
From a different point of view, this invention has a further object of providing for incorporation in a system for soldering printed boards, an apparatus for the application of foam flux which is capable of uniformly and finely foaming a flux thereby forming a foam flux layer possessing a substantially flat and smooth upper surface and allowing the upper surface to contact a printed board being brought in as supported on a conveying device thereby fracturing the foam flux and enabling the scattered droplets of flux from the fractured foam flux to be uniformly applied on the surface of the printed board intended for soldering.